This invention relates to the field of photodetectors.
Existing photon detectors, such as photoconductive detectors, photo tubes and photomultiplier tubes, have relatively low quantum efficiencies. At best, the quantum efficiency is of order 0.3. That is to say, these devices register a small fraction of the photons that fall on the sensitive surface of the detector. The most sensitive photon detectors currently available are photomultiplier tubes. For optimum wavelength and the best photosensitive surfaces, these devices can have quantum efficiencies as high as 0.3. But, for less than optimum conditions, the quantum efficiency is often much smaller. The quantum efficiency is the probability that a photon incident on the sensitive surface of the detector is registered by the detector. Thus the best available photon detectors register about one third of the photons that hit the detector surface. Other types of photon detectors, e.g., photoconductive solid-state devices, have quantum efficiencies substantially less than that of the photomultiplier tube.
Another disadvantage of the photomultiplier tube is its dark current. The dark current consists of a sequence of pulses that is generated by the tube when no light falls on its sensitive surface. The dark current is due to thermionic emission and other physical processes in the detector and associated electronics. The dark current is an unavoidable noise source because the dark-current pulses are indistinguishable from the pulses generated by the detector when photons are detected.